In recent years electric utilities have been developing alternative technologies for power generation to meet the increased demands of society. One alternative technology that is of recent interest is referred to as the Integrated Coal Gasification Combined Cycle (IGCC). In this type of facility, coal is converted into a liquid or gaseous fuel through gasification followed by combustion and expansion of the combusted gases In a turbine. Power is recovered from the turbine. A significant advantage of an IGCC system Is that capacity can be added in stages which permits incremental capital expenditures for providing the additional power demands of society. In that regard, business decisions become easier. Although the IGCC systems permits phasing in terms of providing additional capacity and makes decisions easier from a business perspective, it presents problems to the design engineer because of the inability to match performance and efficiency requirements in the IGCC system.
One of the earlier integrated IGCC systems Involved a cryogenic air-separation system and power turbine and is Swearingen U.S. Pat. No. 2,520,862. The air separation unit was of common design, e.g., it employed a liquefaction and dual column distillation system with the dual column distillation system having a higher pressure and lower pressure column. Low purity, low pressure oxygen generated In the air separation unit was used for oxidizing the fuel with the resulting gases being expanded in the power turbine. To enhance efficiency of the power turbine, waste nitrogen-rich gas was taken from the higher pressure column and mixed with the compressed feed air for combustion. Two problems were presented by this approach, the first being that it was impossible to independently set the pressures of the higher pressure column with that of the inlet pressure to the turbine to achieve an optimum operating efficiency for both the air separation unit and for the power turbine and, secondly, nitrogen separation in the lower pressure column was inefficient due to the lack of nitrogen reflux available for that column.
Coveney in U.S. Pat. No. 3,731,495 disclosed an IGCC comprising an integrated air separation unit and power system wherein the cryogenic air separation unit employed a conventional double-column distillation system. In contrast to Swearingen, Coveney quenched combustion gases with a waste nitrogen-rich gas obtained from the lower-pressure column. However, in that case, it was impossible to independently control the pressure in the lower pressure column and the pressure at the inlet to the power turbine. As a result it was impossible to operate the lower pressure column and the turbine at its optimum pressures.
Olszewski, et al. in U.S. Pat. No. 4,224,045 disclosed an improved process over the Coveney and Swearingen processes wherein an air separation unit was combined with a power generating cycle. Air was compressed via a compressor with one portion being routed to the air separation unit and the other to the combustion zone. In order to nearly match the optimum operating pressures of the air separation unit with the optimum operating pressures of the power turbine cycle, waste nitrogen from the lower pressure column was boosted in pressure by means of an auxiliary compressor and then combined with the compressed feed air to the combustion unit or to an intermediate zone in the power turbine itself. Through the use of the auxiliary nitrogen compressor there was an inherent ability to boost the nitrogen pressure to the combustion zone independent of operation of the air separation unit. By this process, Olszewski was able to more nearly match the optimum pressures for the air separation unit and power turbine systems selected.
One problem associated with each of the systems described above is that even though the air separation units were integrated into an IGCC power generating system, the processes were not truly integrated in the sense that the air separation unit and IGCC power system were able to operate at their optimum pressures independent of each other. Although Olszewski reached a higher degree of independent operability than Coveney and Swearingen, the process scheme was only suited for those processes wherein air was taken from the air compression section of the gas turbine and used for the air feed to the air separation unit. The air inlet pressure to the air separation unit could be varied by using either a turbo expander on the air inlet stream or a booster compressor. Although it was possible to obtain an optimum pressure in the air separation system in the Olszewski process, for example, each prior art process received a part or all of the feed for the air separation unit from the gas turbine compressor section. However, the inlet pressure to the Olszewski air separation unit required a lower-pressure rectification stage having a pressure of at least 20 psi lower than the optimum ignition pressure in the combustion zone. In many cases enhanced operating efficiencies of the lower pressure column in the air separation unit may require a higher operating pressure than available in Olszewski, et al., particularly when moderate pressure nitrogen is desired.